Month: December 1996

What material is used in glass to make it polarize light?

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What material is used in glass to make it polarize light? — FG, Torrance, CA

Actually, the polarizing material you are referring to is a plastic that has been impregnated with iodine atoms. The plastic, polyvinyl alcohol, is heated and stretched to align its long molecules in a particular direction. This plastic is then exposed to iodine, which binds to the long molecules and forms the equivalent of molecular wires along the direction of the aligned plastic molecules. These molecular wires absorb light that is polarized along them because the light’s electric field points along its polarization direction and pushes electric charges wastefully along the iodine wires. This light is absorbed and its energy is converted to thermal energy, leaving only light with the other polarization.

I have heard that microwaving can destroy certain nutrient molecules in food, su…

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I have heard that microwaving can destroy certain nutrient molecules in food, such as vitamins. Is this true? — D, Boulder, CO

A microwave oven heats the food it cooks; nothing more. If it damages nutrients, then it’s by overheating those nutrients. Such overheating could happen in a microwave oven if you don’t move the food about during cooking. That’s because the microwaves aren’t uniformly distributed in the cooking chamber and some parts of the food heat faster than others. Some parts of the food could become hotter than you intend and this overheating could damage sensitive molecules. However, I think that microwave cooking is probably less injurious to the food than conventional cooking. It’s pretty hard to burn food in a microwave!

What is the relationship between turbulence, laminar flow, and Reynolds number?

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What is the relationship between turbulence, laminar flow, and Reynolds number? — DD, SC

The Reynolds number is a measure of the way in which a moving fluid encounters an obstacle. It’s equal to the fluid’s density, the size of the obstacle, and the fluid’s speed, and inversely proportional to the fluid’s viscosity (viscosity is the measure of a fluid’s “thickness”—for example, honey has a much larger viscosity than water does). A small Reynolds number refers to a flow in which the fluid has a low density so that it responds easily to forces, encounters a small obstacle, moves slowly, or has a large viscosity to keep it organized. In such a situation, the fluid is able to get around the obstacle smoothly in what is known as “laminar flow.” You can describe such laminar flow as dominated by the fluid’s viscosity—it’s tendency to move smoothly together as a cohesive material.

A large Reynolds number refers to a flow in which the fluid has a large density so that it doesn’t respond easily to forces, encounters a large obstacle, moves rapidly, or has too small a viscosity to keep it organized. In such a situation, the fluid can’t get around the obstacle without breaking up into turbulent swirls and eddies. You can describe such turbulent flow as dominated by the fluid’s inertia—the tendency of each portion of fluid to follow a path determined by its own momentum.

The transition from laminar to turbulent flow occurs at a particular range of Reynolds number (usually around 2500). Below this range, the flow is normally laminar; above it, the flow is normally turbulent.

Could you explain the microscopic model of temperature in a gas?

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Could you explain the microscopic model of temperature in a gas? — DD, SC

Thermodynamics imposes a severe constraint on the meaning of temperature by observing that when two objects are at the same temperature, no heat flows between them when they touch. That constraint leads to the follow possibility: in a gas composed of independent particles, temperature must be proportional to the average internal kinetic energy per particle. By internal kinetic energy, I mean that we are excluding any kinetic energy associated with the movement of the gas as a whole. And by average per particle, I mean to add up all the internal kinetic energies and divide the sum by the number of particles. With this definition of temperature, two bodies of gas that have the same temperature won’t exchange heat when they touch. It turns out to be a good definition of temperature and the one that we use in general.

A friend was telling me of a guy who created a TV satellite dish out of chicken …

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A friend was telling me of a guy who created a TV satellite dish out of chicken wire in his attic — how would you do it, adjust it, and what kind of home-brew receiver would be required to use it? – BP

Since the microwaves used in satellite transmissions have wavelengths of several centimeters or more, they can’t pass through holes in a conducting material if those holes are less than about a centimeter in diameter. As a result, chicken wire reflects microwaves as though it were a sheet of solid metal. You can form a dish antenna by bending chicken wire into a parabola. When the microwaves from the satellite strike this parabolic reflecting surface, they are brought together to a focus at a particular point above the center of the parabola. If you then place a microwave receiving device at this focal point, you’ll be able to watch satellite TV.

If you want to do this, you should make a cardboard template for the parabolic shape and bend the chicken wire carefully to match this template. The more highly curved the parabola, the closer the focus will be to the dish’s surface. You should aim this dish directly at the satellite and put the receiving unit at the focus of the parabola, above its center. However, you’ll have difficulty building the receiving device yourself, although there are probably kits you can buy. The receiver should have a tiny antenna, a microwave amplifier, and a frequency down-converter, all together on a single circuit board. Working with microwave-frequency electronics is difficult because the wave character of the electric signals is painfully obvious in those circuits. Designing microwave circuits is a job for experts. In short, you can build the dish, but you should buy the receiver that sits at the center of the dish.

What is the “optimal” weight distribution for a pinewood derby car

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What is the “optimal” weight distribution for a pinewood derby car — in front/behind, above/below the center of gravity? – BP

I’ll assume that the car starts on a slope and coasts downhill to a level finish. If that’s the case, then you want to put the car’s center of gravity as far back in the car as you can get it. That way, the center of gravity will start as high as possible in the tilted car and will finish as low as possible in the level car. During a race, the car obtains its kinetic energy (its energy of motion) from its gravitational potential energy. The farther the car’s center of gravity descends during the race, the more gravitational potential energy will be converted to kinetic energy and the faster the car will go.

What is the “optimal” shape for a pinewood derby car

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What is the “optimal” shape for a pinewood derby car — I’m guessing some sort of short, flat, thin rectangle. – BP

The car’s biggest obstacle is air resistance, which in this case is a force known as “pressure drag.” The pressure drag force is proportional to the size of the turbulent wake the car creates in the air as it passes through the air. Streamlining is important to minimizing this wake. The thinner and shorter you can make the car, the smaller its wake will be. The ideal shape would be an airfoil, like those used in airplane wings and bodies. These carefully tapered shapes barely disturb the air at all and experience very little pressure drag. If you design your car to resemble a wingless commercial jet airliner, you will be doing pretty well.

Hydrogen atoms can form a single bond to each other, oxygen atoms can form a dou…

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Hydrogen atoms can form a single bond to each other, oxygen atoms can form a double bond to each other, and nitrogen atoms can form a triple bond to each other. Is there any element that can form a quadruple bond? — KC, Mendenhall, MS

The bonds that you are referring to are call “covalent bonds,” in which two atoms share a pair of electrons in order to lower their total energy. When two electrons are shared in this manner, the electrons are able to spread out over two atoms rather than one. This broadening of their territories lowers their kinetic energies because of quantum mechanical effects. The electrons also spend large portions of their times between the atoms, where they lower the electrostatic potential energies of the two atoms. Lowering the total energy of the two atoms binds them together.

The number of covalent bonds that form between two atoms depends on the number of electrons in those atoms. Hydrogen atoms have only one electron each and can form only one covalent bond. Oxygen atoms have two electrons each that they can share and form two covalent bonds. Nitrogen atoms have three electrons to share and form three covalent bonds. And carbon atoms have four electrons to share, so you might expect them to form four covalent bonds. But there’s a hitch…

In the first covalent bond that forms between two atoms, the pair of electrons positions itself directly in between the atoms. This arrangement is most effective for lowering the energy of the system and binding the two atoms together. Chemists call this arrangement a “sigma bond.” In the second covalent bond, the two electrons position themselves on both sides of the sigma bond. If you picture the atoms as two people facing one another and holding hands, the electrons are located along the arms of the two people. This arrangement is reasonably effective for lowering the energy of the system and is called a “pi bond.” The third covalent bond is also a pi bond, but it forms 90° from the first pi bond, as though the two people are now touching their heads and their feet together along with their hands. With a sigma bond and the two pi bonds between the atoms, there is no room for additional electrons. The fourth covalent bond that two carbon atoms would like to form with one another simply can’t form. While two carbon atoms will bind together with a triple bond, each atom will have one remaining electron that is still seeking a partner. The carbon dimer molecule is a highly reactive double radical that will bind to just about anything it encounters.

Does the pull of the moon have any effect on a person’s behavior?

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Does the pull of the moon have any effect on a person’s behavior? — PSC, Summerville, WV

No, but for an interesting reason. While the moon’s gravity acts on people, it also acts on everything around them and everything falls toward the moon at the same rate. Because of this uniform falling, we don’t feel the moon’s gravity at all. This effect is identical to the one that astronauts feel as they orbit the earth—the earth’s gravity pulls on them and on their spaceship, but they are falling freely under the influence of that gravity and they don’t feel it—they feel weightless. Since we are falling freely under the influence of the moon’s gravity, we don’t feel it either—we feel moon-weightless.

Since we are being pulled toward the moon by the moon’s gravity, you might wonder why we don’t crash into the moon. That’s because we’re traveling sideways so fast that we perpetually miss the moon and circle it once every 27.3 days. Similarly, the moon perpetually misses the earth and circles it, too.

The only significant effect of the moon’s gravity is to create the tide. The earth’s oceans are so large that they’re sensitive to variations in the moon’s gravity. The moon’s gravity decreases with distance from the moon, so that the oceans on the near side of the earth are pulled harder than the oceans on the far side of the earth. The result is two bulges in the oceans—one on the near side of the earth and one on the far side of the earth. These bulges create the familiar high and low tides that we observe at the seashore.

How does an internal combustion engine work?

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How does an internal combustion engine work? — RT, Kitchener, Ontario

An internal combustion engine burns a mixture of fuel and air in an enclosed space. This space is formed by a cylinder that’s sealed at one end and a piston that slides in and out of that cylinder. Two or more valves allow the fuel and air to enter the cylinder and for the gases that form when the fuel and air burn to leave the cylinder. As the piston slides in and out of the cylinder, the enclosed space within the cylinder changes its volume. The engine uses this changing volume to extract energy from the burning mixture.

The process begins when the engine pulls the piston out of the cylinder, expanding the enclosed space and allowing fuel and air to flow into that space through a valve. This motion is called the intake stroke. Next, the engine squeezes the fuel and air mixture tightly together by pushing the piston into the cylinder in what is called the compression stroke. At the end of the compression stroke, with the fuel and air mixture squeezed as tightly as possible, the spark plug at the sealed end of the cylinder fires and ignites the mixture. The hot burning fuel has an enormous pressure and it pushes the piston strongly out of the cylinder. This power stroke is what provides power to the car that’s attached to the engine. Finally, the engine squeezes the burned gas out of the cylinder through another valve in the exhaust stroke. These four strokes repeat over and over again to power the car. To provide more steady power, and to make sure that there is enough energy to carry the piston through the intake, compression, and exhaust strokes, most internal combustion engines have at least four cylinders (and pistons). That way, there is always at least one cylinder going through the power stroke and it can carry the other cylinders through the non-power strokes.